The present invention relates to a magnetron sputtering apparatus and method having a ring-shaped flat target.
Magnetron sputtering technology has been used to deposit thin films on substrates. Because of its capability of high-speed processing at low temperatures, magnetron sputtering is executed in most of the current film forming apparatuses. According to the magnetron sputtering, ions of plasma generated in the vicinity of a target by discharging or the like are let to collide with the target, thereby sputtering and adhering particles to a substrate and forming a thin film on the substrate.
A conventional example will be described with reference to FIGS. 9, 10, and 11.
FIG. 9 is a sectional view of a cathode part of a conventional magnetron sputtering apparatus using a ring-shaped flat target. The cathode part is symmetrical with respect to a central axis 1. Magnets 3 are arranged at a rear surface of a ring-shaped flat target 2, which form magnetic fields 4. With the thus-constituted cathode part set in a vacuum chamber in a manner that a front surface of the target 2 faces a to-be-processed substrate, when the electricity is supplied to the target 2 from a high voltage power source for glow discharge after a sputtering gas is introduced into the chamber, a high-density plasma enclosed by lines of magnetic force is generated. As ions in the plasma collide against the front surface of the target, atoms of the target are sputtered to adhere to a confronting surface of the substrate, thereby forming a thin film on the substrate. A speed to sputter/erode the target 2 is fast in the vicinity of an area 5 where the plasma is in a high density, and is locally accelerated in the vicinity of an area 6.
FIG. 10 is a sectional view of the sputtered target including the central axis 1 when the target of FIG. 9 is used to its using limit. A half of the target symmetric to the central axis is abbreviated in FIG. 10. Reference numerals in FIG. 10 indicate: 9 an original shape of the target before being sputtered; 10 a sputtered part of the target; and 11 a part left without being sputtered. As is apparent in FIG. 10, the target is most sputtered in the vicinity of a D point, resulting in the formation of a V-shaped sputtered eroded surface. In the case where the sputtering is carried out in the above manner to cause the above V-shaped sputtered surface, a voluminal using efficiency of the target, namely, a volume ratio of the sputtered part of the target to the target before being sputtered (including the volume inside its inner periphery) when the target is used to the using limit is approximately 20%. In other words, the target which is expensive is impossible to fully utilize in the conventional arrangement. Moreover, a film forming speed or a film thickness distribution disadvantageously changes with time when the target is sputtered with an irregular erosion accompanied as in FIG. 10.
For solving the aforementioned problems, new technical ideas have been actually discussed as in Japanese Laid-Open Patent Publication Nos. 5-209266 (209266/1993) and 5-179440 (179440/1993). A cathode for magnetron sputtering disclosed in Japanese Laid-Open Patent Publication No. 5-209266 has magnets arranged at a rear surface of a target similarly to the conventional example of FIG. 9, and also ferromagnetic bodies are provided both in an outer and in an inner peripheries of the target, so that a magnetic flux spreads over a front surface of the target. Accordingly, a plasma density is averaged and consequently the target is more smoothly and uniformly sputtered/eroded.
FIG. 11 is a sectional view showing a sputtered target when the target is used in the cathode for magnetron sputtering of Japanese Laid-Open Patent Publication No. 5-209266 to the using limit. Similar to FIG. 10, reference numerals in FIG. 11 respectively represent: 9 an original shape of the target before sputtering; 10 a sputtered part thereof; and 11 a part left without being sputtered. In comparison with FIG. 10, the sputtering is more uniformly proceeded in FIG. 11, whereas the sputtering speed is locally increased in the vicinity of an E point. The using efficiency of the target in terms of volume at this time including the volume inside the inner periphery of the target is near 40%.
In the prior art wherein the magnets are set at the rear surface of the target, the sputtering generates the V-shaped sputtered surface as shown in FIG. 10. The target becomes locally thin if the sputtering proceeds in this manner to form a V-shaped surface, and is eventually impossible to use beyond its using limit, in other words, as soon as the thickness of the locally thin part reaches a predetermined value or smaller even if a sufficient amount of material to form thin films remains in the target. The target which is expensive is also impossible to fully utilize in this arrangement. If the sputtering is continued locally fast, the distribution of sputtered particles is changed with time, thus changing an adhering rate of particles to the substrate. As a result, even when the thickness of the film formed on the substrate in an early stage of the sputtering is uniform, the thickness is unfavorably turned not uniform in a final stage.
As indicated in FIG. 11, the local erosion by sputtering is given rise to in the vicinity of the E point even in the arrangement proposed in Japanese Laid-Open Patent Publication No. 5-209266, whereby the voluminal using efficiency of the target is decreased and the thickness of the thin film adhering to the substrate is rendered irregular.
In a diagram of FIG. 8, a change with time of the thickness distribution of a thin film formed on a substrate of a 40 mm inner diameter and a 120 mm outer diameter is represented with respect to integrating watts. An axis of ordinate indicates a relative film thickness when an edge at an inner periphery of the substrate has a thickness of 1, and an axis of abscissa shows a distance from a central point of the substrate. As is clear from FIG. 8, the thickness distribution changes large with time in the conventional example.